Ito Daisuke, Suzuki Norihiro
Department of Neurology, School of Medicine, Keio University, Shinjuku-ku, Tokyo, Japan.
Ann Neurol. 2007 Mar;61(3):237-50. doi: 10.1002/ana.21070.
Heterozygous mutations in the Seipin/BSCL2 gene have recently been identified in two autosomal dominant motor neuron diseases, distal hereditary motor neuropathy type V and Silver's syndrome. Seipin protein is reportedly a transmembrane protein localized in the endoplasmic reticulum (ER). N88S and S90L mutations of this protein disrupt its glycosylation, resulting in its aggregation, but the mechanism of neurodegeneration remains unclear. To clarify the molecular pathogenesis of seipin-related motor neuron diseases, we expressed wild-type and mutant seipin proteins in neuronal and nonneuronal cells.
Coexpression of human seipin and ubiquitin showed that seipin is polyubiquitinated and its ubiquitination is enhanced by mutation. Treatment of cells with a proteasome inhibitor increased the amounts of mutant seipin in the cells, suggesting that they are degraded through the ER-associated degradation pathway. Immunoprecipitation studies showed that mutant seipin stably binds to the ER chaperone calnexin, indicating accumulation of unfolded mutant seipin in the ER. Furthermore, expression of mutant seipin increased the level of ER stress-mediated molecules and induced apoptosis in cultured cells.
These findings demonstrate that seipin/BSCL2-related motor neuron diseases are novel conformational diseases, and we suspect that they are tightly associated with ER stress-mediated cell death.
最近在两种常染色体显性运动神经元疾病——Ⅴ型远端遗传性运动神经病和西尔弗综合征中发现了Seipin/BSCL2基因的杂合突变。据报道,Seipin蛋白是一种定位于内质网(ER)的跨膜蛋白。该蛋白的N88S和S90L突变破坏其糖基化,导致其聚集,但神经变性的机制仍不清楚。为了阐明Seipin相关运动神经元疾病的分子发病机制,我们在神经元和非神经元细胞中表达野生型和突变型Seipin蛋白。
人Seipin和泛素的共表达表明Seipin被多聚泛素化,且其泛素化因突变而增强。用蛋白酶体抑制剂处理细胞增加了细胞中突变型Seipin的量,提示它们通过内质网相关降解途径被降解。免疫沉淀研究表明,突变型Seipin与内质网伴侣钙连蛋白稳定结合,表明未折叠的突变型Seipin在内质网中积累。此外,突变型Seipin的表达增加了内质网应激介导分子的水平,并诱导培养细胞凋亡。
这些发现表明,Seipin/BSCL2相关运动神经元疾病是新型构象疾病,我们怀疑它们与内质网应激介导的细胞死亡密切相关。